Rechargeable batteries and methods of making same

ABSTRACT

Systems and methods for rechargeable batteries are provided. In an embodiment, a battery may include a cathode, an anode, an electrolyte solution, and a current collector. The anode may include a 3D porous structure. The 3D porous structure may have a higher electrical conductivity at one end than at the other end, and lithium ions may be dispersed throughout the 3D porous structure.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.16/802,061, filed Feb. 26, 2020, which claims priority to ProvisionalPatent Application No. 62/810,439, filed Feb. 26, 2019, the disclosuresof which are incorporated herein in their entirety.

FIELD OF THE INVENTION

The present invention relates to electrochemical energy storage devices,such as batteries which contain a lithium metal anode.

BACKGROUND OF THE INVENTION

Lithium ion batteries since their introduction have doubled in specificenergy to over 250 Wh/kg. These batteries have found widespreadapplication in portable electronics and mobile communications devices aswell as in, for instance, HEVs, PHEVs and EVs. The specific energy ofthe lithium ion battery has nonetheless reached a plateau—and room forimprovement has been limited. Researchers are increasingly focusingtheir attentions on battery chemistries beyond lithium ion batteries.Among these, rechargeable lithium metal batteries, where lithium metalis used as the anode, have been considered as promising candidates fordevelopment and commercialization. However, the lithium metal anode,although having high specific capacity (3860 mAh/g), still suffers fromissues of dendrite growth, solid electrolyte interface (“SEI”)instability and volume change during cycling. While great effort hasbeen made to try to address these issues, the successful solution is yetto emerge.

Lithium sulfur batteries have been viewed as promising candidates due tothe use of abundant and low cost sulfur material (sulfur price iscurrently at roughly $0.57/kg with 10,000,000 tons/year production asthe oil refinery by-product), and most importantly, an extremely hightheoretical specific energy of 2680 Wh/kg (S. Zhang, J. Power Sources,2013, 231, 153; S. Zhang, Energies, 2012, 5, 5190)—about 4.6 timeshigher than that of the conventional lithium ion battery. However, evenwith over three decades of R&D effort, commercial Li/S batteries haveyet to be introduced into the market due to many challenges relating tothe battery chemistry. These challenges include short cycle life,especially for sulfur electrodes with high areal loading over 5 mAh/cm²,low cycling efficiency, high self-discharge rate due to polysulfiteshuttling, as well as issues associated with lithium metal anodes.

Another candidate is a battery with a lithium metal anode and a highspecific capacity cathode, such as that shown athttp://www.greencarcongress.com/2009/12/panasonic-20091225.html andhttp://www.solidenergysystems.com/technology.html) (with specificcapacity over 200 mAh/g). However, the development of a high performancelithium metal anode is the key to the success of such batteries.

While a lithium metal electrode is routinely used in primary lithiumbatteries, it has not been successfully commercialized in rechargeablebatteries due to problems with lithium dendrite growth and SEIinstability. In fact, a fire problem caused the eventual failure of thevery first lithium metal based battery developed by Exxon (K. Nishio, N.Furukawa, in Handbook of Batteries Materials, Ed. J. O. Besenhard, p.56, Wiley-VCH, Weinheim, 1999). The problems hindering the developmentof rechargeable lithium metal batteries can be now traced to three mainissues.

The first issue is the formation and growth of lithium dendrites duringrepeated cycling, leading to cell shorting and causing safety concerns.Such dendritic lithium growth could also reduce the charging efficiencyand causes dead lithium formation which leads to voltage drop.

The second issue is related to the instability of the SEI layer on thelithium anode in an organic electrolyte; side reactions can consume theelectrolyte and lead to cell failure.

The third problem is related to the volume change during lithiumdeposition and stripping. This issue becomes more pronounced for highareal capacity lithium anodes (>12 mAh/cm²) and for discharging at veryhigh rates.

A tremendous amount of theoretical and experimental work has gone intothe development of an understanding of lithium dendrite formation andits prevention and it is now widely recognized that rechargeable lithiumbatteries with conventional liquid electrolytes do not work. Theoreticalwork by Newman and his coworker found that polymer electrolytes or otherSEI layer with a critical modulus (>ca. 6 GPa) value can effectivelyblock lithium metal dendrite growth. C. Monroe, J. Newman, J.Electrochem. Soc., 2005, 152, A396.

Efforts have been devoted to mitigating the problem on lithium metal byusing electrolyte additives, artificial SEI layer formation or surfacecoating, lithium metal alloy anode, solid state electrolytes and 3Dlithium anodes. The ionic conductivity of solid state electrolytesincluding polymer electrolytes and inorganic electrolytes has beensignificantly improved in recent years. However, due to their solidnature, the contact resistance between solid state electrolytes and theelectrode is still high—limiting their usage in high power applications.Electrolyte additives, such as organic solvent FEC, Cs⁺ salt and LiNO₃were found to be effective in obtaining smooth lithium deposition orincreasing the Coulombic efficiency. However, the SEI layer formed inthe electrolyte is effective only at low areal capacity and the SEIlayer tends to crack with high capacity lithium stripping, leading toearly failure. Artificial SEI formation on the lithium surface can beemployed to tailor an SEI layer which can be more effective. Nanometerthick inorganic Li₃PO₄ SEI has shown lower impedance than Li metalwithout SEI coating; the electrode also demonstrated up to 1 mA/cm²without SEI layer cracking. However, at 2 mA/cm², the SEI layer tends tocrack during cycle. Recently, Yi Cui's group (D. Lin, Y. Liu, W Chen, G.Zhou, K. Liu, B. Dunn, Y. Cui, Nano. Lett., 2017, 17, 3731) demonstrateda gas phase method to create artificial inorganic LiF SEI layer byreacting lithium metal with Freon gas at 150° C. The SEI protected Liexhibited good cycle stability in Li/S cells. However, a pure inorganicSEI layer still has stability issues due to large volume change for Limetal anode with high areal loading. Moreover, the preparation methodinvolves very active molten lithium metal and is hard to implement inpractical applications.

Recently, three dimensional (3D) lithium metal electrodes wereinvestigated to reduce the current density of the lithium metal, toinhibit lithium dendrite growth, and to accommodate Li volume changeduring battery cycling. The electrode has shown much better powerperformance, cycle stability and coulombic efficiency.

3D structural engineered lithium anodes have become a very promisingstrategy to solve not only the dendrite growth issue but also toincrease the areal loading of the lithium anode without compromising thecycle performance. In these 3D structures, both conductive frameworks,including carbon-based frameworks and metal-based frameworks, andnon-conductive frameworks, such as polyimide non-woven paper, wereemployed to host the lithium deposition.

Mukherjee et al. (R. Mukerjee, A. V. Thomas, D. Datta, E. Singh, J. Li,O. Eksik, V. B. Shenoy, N. Koratar, Nat. Comm., 2014, 5, 3710) usedporous graphene networks as the lithium anode host. The electrodeexhibited over 900 mAh/g capacity and over 1000 charge/discharge cycleswith coulombic efficiency over 99%. Nanostructured lithium metal anodesbased on graphene also showed very high stable areal capacity of 5mAh/cm². By using rGO film which contains rich lithiophilic groups orusing CVD Si coating porous carbon scaffold, Cui et al. (Z. Liang, D.Lin, J. Zhao, Z. Lu, Y. Liu, C. Liu, Y. Lu, H. Wang, K. Yan, X. Tao, Y.Cui, PNAS, 2016, 113, 2862) was able to infuse Li metals thermally intothe 3D structure. The electrode with Si coated scaffold has demonstrateda very high stable capacity of 2000 mAh/g at low areal loading of 1mAh/cm², which could not be used in the practical high energy densitybatteries. Free standing hollow carbon fiber felt was employedpreviously by Liu et al. (L. Liu, et al., Joule, 2017, 1, 563) to makethe 3D lithium anode. In their processes, impregnation of metal lithiuminside the 3D structure was made through electrochemical deposition.Moreover, due to lower electronic conductivity, the electrode cannot beused in making standard cylindrical or pouch type rechargeablebatteries. Other carbon fibers or carbon nanofibers were also employedin making the 3D lithium anodes. However, high surface area carbon couldsignificantly increase the contact area between lithium metal andelectrolyte, resulting in consumption of the electrolyte in SEI layerformation. Additionally, poor mechanical stability and low electronicconductivity could be an issue for making commercial lithium batteries.

FIG. 1 is a schematic illustration of Li deposition processes into a 3DCu nanowire substrate.

Metal based 3D structures, such as nanostructured copper framework,stainless steel fiber felt and fibrous Li₇B₆ matrix were employed inmaking a 3D lithium anode. For example, a free-standing 3D coppernanowire network was successfully prepared and used as the 3D Li anodecurrent collector. Up to 7.5 mAh/cm² areal capacity of lithium can bedeposited into the current collector without dendrite growth. Theelectrode also exhibited 98.6% coulombic efficiency for 200 cycles at 1mA/cm² rate—far better than 50% efficiency in 50 cycles for Li anode onplanar copper current collector. A rate of about 10 mA/cm² was alsodemonstrated. 3D anodes seem to be able to solve the areal capacityissue, rate issue, volume change issue and dendrite growth issue.

Nonetheless, by carefully analyzing these results, one can see that allof these approaches involved costly material and processes. Mostimportantly, the top-down lithium deposition cannot be sustained overmany cycles and dendrite growth formation occurs on the currentcollector.

As confirmed by SEM observation and due to the high electronicconductivity of the 3D current collector, the top and bottom of the 3Dcurrent collector have the same electrochemical potential. However, thetop part of the current collector has fast access to the Li ion (shortdiffusion pathway from the electrolyte), so lithium deposition will takeplace on the top of the current collector as shown in FIG. 1. Over time,lithium deposition will push downward to the bottom. Over long cycles,the channel for Li ion diffusion could be blocked and the lithiumdeposition will continue on the top part, which could lead to bypass 3Dcurrent collector and form lithium dendrites on the top part—defeatingthe advantage of the 3D structure.

In another prior art (Q. Yu, Y. B. He, W. Lv, Y, Zhao, B. Li, F. Kang,Q. H. Yang, Adv. Mater., 2016, 28, 6932), a 3D porous current collectorwas prepared by etching off zinc from a brass foil. Such a 3D structurewas fragile to handle. Lithium was also introduced by electrochemicaldeposition of lithium metal into the 3D structure. Similar to theprevious 3D structures, the lithium tends to grow on top of the currentcollector instead inside the 3D current collector.

Commercial nickel and copper foam has also been used to make the 3Dlithium anode. The electrode suffered the same problem as all metalbased 3D current collectors due to high electronic conductivity on thesurface, which could promote preferential lithium deposition on top.Additionally, the thinnest commercial nickel foam is over 200 μm, whichmay be too thick in high energy density battery applications.

Another practical problem in the art is associated with the methods topre-lithiate the 3D structure. For Li/S or Li/Air batteries and evenLi/metal oxides (e.g. LiCoO₂ or other metal oxide based cathodematerials) batteries, pre-lithiated anodes are needed. Electrochemicaldeposition on 3D current collectors has been employed, but is notfeasible in practical applications due to cost concerns. The thermalinfusion method through molten lithium was widely used as thealternative in the literature. There are three issues associated withthis process. First, it is very hard to scale up the process forindustrial applications due to the inert environment requirement, e.g.glove box filled with inert gas, and great safety concerns. Secondly,the amount of the infused lithium cannot be well controlled. The lithiummetal usually will occupy all of the void volume in 3D currentcollector, which is not a problem for Li/S or Li/Air batteries since thefirst step in those batteries is discharging, and Li metal will beconsumed. However, for Li/LiCoO₂ or Li/LiFePO₄ batteries, the first stepis charging, where more Li metal will be deposited onto the 3D currentcollector. If the 3D structure is completely filled with Li, the extraLi metal will be deposited outside the 3D structure, leading to agreater tendency of dendrite growth. Also, an excess amount of Li couldreduce the battery's specific energy. Thirdly, high temperature moltenlithium could possibly damage 3D current collectors, and preclude use ofcertain polymers.

Pure polymer based 3D hosts were also investigated. Liang et al. (Z.Liang, et al., Nano Lett., 2015, 15, 2910) used 3D polyacrylonitrilenanofiber felt as the lithium metal host. The polymer host solved thevolume change problem of lithium anode during cycling. However, theelectronic conductivity of the hosts is much weaker than that of carbonand metal 3D hosts.

In summary, current state-of-the-art 3D hosts showed promisingperformance for making 3D lithium metal anodes. However, they stillsuffer short cycle life at high areal loading, and problems in practicalapplications, including high cost, and difficulty in pre-lithiation.

SUMMARY OF THE INVENTION

It is thus an object of the present invention to provide an anode with amultiple layer 3D structure having a conductivity gradient throughoutthe structure. The bottom end of the anode (close to lithium) will havehigher electronic conductivity and the top end of the anode (close toelectrolyte) will have lower electronic conductivity, such that the Lideposition will take place from the bottom to top.

In order to overcome the drawbacks of current state-of-the-artrechargeable lithium metal batteries, innovative 3D lithium anode willbe introduced. This will be discussed in the following sections.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a set of diagrams depicting a prior art lithium anode;

FIG. 2 is a schematic illustration of a 3D lithium anode, in accordancewith various embodiments;

FIG. 3 is a schematic illustration of a 3D lithium anode, in accordancewith various embodiments;

DETAILED DESCRIPTION

FIG. 2 is a schematic illustration of a 3D lithium anode, in accordancewith various embodiments. In one embodiment, a 3D lithium anode 200 iscoupled to an aqueous electrolyte solution 202 and may include a 3Dconductive porous structure 204, a lithium metal layer 206, and a thinmetal current collector 208. Current collector 208 may include aconventional anode current collector, including, but not limited to,copper, copper alloy, nickel, nickel alloy, stainless steel withthickness from 1 μm to 200 μm and more preferably, 5 μm to 10 μm.Lithium metal layer 206 may be made of pure metallic lithium with athickness ranging from 5 μm to 200 μm. Lithium may be laminated orextruded on current collector 208 by any method known in the art. 3Dporous structure 204 may have a thickness ranging from 5 μm to 500 μm,and more preferably from 10 μm to 150 μm, and most preferably from 20 μmto 80 μm.

In an embodiment, 3D structure 204 may have a porosity ranging from 30%to 95%, and more preferably from 60% to 90%. 3D structure 204 may bemade of conductive materials in the form of particles or fibers or anyother shapes, and polymer binders, including but not limited to PVdF, orpolyimide. The electronic conductivity of 3D structure 204 may bedesigned to have high electronic conductivity at the bottom of the layerand lower electronic conductivity at the top of the layer by adjustingthe conductivity of the material compositions. The conductive materialsused may include conductive carbon and conductive metals (such ascopper, nickel). By varying the ratio of carbon and metal in theformula, the conductivity of 3D structure 204 can form a gradient sincemetals have higher specific conductivity than carbon. Further, carbonand metals can be coated or doped with other lithiophilic elements,including but not limited to Sn, Zn, Ag, Bi, In, Ga, Al, N, P, Si, Ge,or alloys of these elements. In preparation of 3D structure 204,conductive materials, polymer binder solution and/or pore former may bemixed to form a coating slurry, which may subsequently be casted on asubstrate such as glass or release liner, followed by drying andremoving of pore former and perhaps calendaring. A stand-alone 3D layermay thus be made. Such 3D structure 204 may be laminated onto metal/Lilayer 206 or simply placed on top of the lithium layer 206 to make the3D lithium anode for rechargeable lithium metal batteries.

FIG. 3 is a schematic diagram of a 3D lithium anode, in accordance withvarious embodiments of the invention. In contrast to the anode shown inFIG. 2, the anode of FIG. 3 may lack a thin metal current collector, butmay otherwise include all other structures as shown in FIG. 2.

Due to the conductivity gradient, different from any other 3D lithiumanode, the lithium deposition may start from the bottom to top, insteadof from the top to the bottom. The lithium stripping during celldischarging may thus start from the top to the bottom. This process maythus prevent lithium dendrite growth on top of 3D current collector.Moreover, doping and coating with lithiophilic elements can facilitatethe smooth growth of lithium on top of 3D layer.

Unlike in the thermal lithium infusion method and the electrochemicalplating method, in this method lithium foil can provide the lithiumsource.

Example 1. Zinc Doped Copper Powder

In an embodiment, a method of forming a lithium anode may be provided.In accordance with various embodiments, a brass powder with particlesize about 10 μm to 50 μm was first washed with dehydrated alcohol toremove surface impurities. Then the powder was immersed in 1 M HCl and 2M ammonium chloride water solution at elevated temperature of 50° C.After a couple of hours, the powder color changed from golden color tocopper color. The power was then filtered and rinsed with copious water,followed by drying in the oven. The collected powder can be useddirectly to make the 3D structure or can be ball milled to reduce thesize of the particle. By controlling the etching time, the residualamount of Zinc in the powder can be adjusted. The lithiophilic zinc canfacilitate the uniform lithium deposition on 3D substrate.

In another embodiment, porous copper powder may be produced withwell-established methods by etching of Zn or Mn from brass alloy or CuMnalloy powders. With controlled alloy composition and etching condition,nanoporous copper with different pore sizes can be made (as shown inFIG. 2a ). After ball milling, 5˜10 um size copper powder with submicronpores can be obtained. Such porous copper powder along with binder maybe mixed and coated on the copper foil followed by drying, additionalporous carbon layer coating, drying and calendaring. 3D currentcollector can thus be made with conventional coating process. In orderto improve the lithiophilicity, residual zinc in copper throughincomplete etching of brass will be used in this embodiment.

Example 2. Evaporation of Zinc on Nickel Powders

Nickel filament powder T255 from Vale with primary particle size of2.2˜2.6 μm can be surface coated with thermal evaporation processes asfollows:

Mix 2 gram of metallic zinc flake and 1 gram of activated carbon in aceramic boat.

Evenly spread 10 grams of nickel powders (Novamet Ni 255) on a pressedNi sponge.

Place the Ni sponge on the mixed zinc flake and activated carbon.

Cover Ni powders with another Ni sponge.

Insert the ceramic boat into the center of a quartz tube inside thetubular oven.

Flush the quartz tube with forming gas (5 vol. % H₂ in N₂) for 10minutes.

Increase temperature to 600° C.˜700° C. at a rate of 5° C. per minute.

Keep the desired temperature for up to 30 minutes.

Cool down to room temperature.

The weight percentage of zinc on nickel powders are about 5 wt % to 15wt %, depending on the annealing temperature and time.

The zinc coated Ni powder was then employed to make the 3D structure.

Example 3. Zinc Coated Nickel Powder

Sol-Gel coating process can also be used to coat the zinc oxide surfacelayer on nickel particles, followed by thermal reduction in forming gas,specifically:

Prepare Zn-containing sol solution in a 100 mL glass beaker by mixingzinc precursor (zinc acetate or zinc nitrate) and base (butylamine orammonium hydroxide or mixed) in organic solvent (ethanol or 2-propanol).

Stir the sol solution for up to 5 hours at a mild temperature of 60-70°C.

Add 10.0 grams Ni powders (Novamet, Ni 255) into the sol solution.

Stir the mixed dispersion in a hood for more than 8 hours, until thecompletely evaporation of the organic solvent.

Transfer the Zn-precursor coated Ni powders into a ceramic boat andinsert it into the center of a quartz tube inside the tubular oven.

Flush the quartz tube with forming gas (5 vol. % H₂ in N₂) for 10minutes.

Increase temperature to 80° C. at a rate of 2° C. per minute and thenkeep the temperature for 4 hours.

Increase temperature to 550-650° C. at a rate of 1° C. per minute andthen keep the temperature for up to 30 minutes.

Cool down to room temperature.

The weight percentage of zinc on nickel powders are about 5 wt % to 15wt %, depending on the ratio of Zn precursor, final annealingtemperature and time.

Zinc coated nickel powder thus made can be used to make 3D conductivestructure for lithium anode.

Example 4. Doped Carbon Black

Zinc doped carbon black was prepared according to the followingprocedures:

Carbon black is soaked in zinc nitrate solution in weight ratio of 95:5.

Solution is dried.

Dry powder is heated up in tube furnace under flow of forming gas at320° C. to decompose zinc nitrate for 1 hour; The temperature is thenramped up to 600° C. and kept at 600° C. for 2 hours.

After cooling down, zinc doped carbon black can be obtained.

The method of making nitrogen doped carbon black is well known in theart.

Example 5. 3D Structured Porous Nickel Film

Porous 3D structure can be made by following procedures:

Make 10% Matrimid solution by adding 1.02 g (Matrimid 5218, Huntsman) ina 4-ounce plastic container with 9.01 g NMP (1-methyl-2-pyrrolidinone,99.5% Sigma-Aldrich) under vigorous stirring, at room temperatureovernight.

Make precursor by adding 4.0 gram Zn-coated Ni powder (Ni 255 coatedwith Zn) in the above container with 10.0 gram 10% Matrimid solution.

Mix the precursor with centrifuge mixer (Speed Mixer, DAC 150 FVZ) at2700 rpm for 2 min, three times.

Cast the precursor on clean glass plate with 5 and 10 mil casting blade,respectively

Then place the glass plates in a hood for 4-6 hour at room temperature

Merge the plates into warm water (40-50° C.), and gently peel off thefilms from the glass plate.

Dry the films for 1 day at room temperature

The thickness of the films are 33 μm, 55 μm, respectively

The thickness of the film can be varied by changing the gap of doctorblade. Other polymer resins, including but not limited to polyamide,polyamide imide, polyvinylidene fluoride, polyether ether ketone, canalso be used to replace Matrimid. Pore former can also be included tochange the porosity of the film.

Example 6. 3D Structure with Conductivity Gradient

Similar to Example 5, mixed N-doped carbon black and Zn coated nickelpower were employed along with polymer binder. A thin layer with highpercentage of zinc coated nickel powder was casted and dried, which wasfollowed by additional casting with formula containing high percentageof N-doped carbon (lower electronic conductivity). After peeling off anddrying, 3D structured porous film with conductivity gradient can bemade. The layer with high metal content and higher conductivity will beplaced directly on top of lithium metal foil or lithium metal foil oncopper current collector to make 3D lithium anode.

Similar to Example 5, mixed N-doped carbon black and Zn doped copper(made from Example 1) were employed along with polymer binder. A thinlayer with high percentage of Zn doped copper powder was first casted,followed by a layer of coating with high percentage of N-doped carbonblack. After peeling off and drying, 3D structured porous copper filmwith conductivity gradient can be made. 3D lithium anode can be madesimilarly.

Example 7. Porosity Control of 3D Structure

Similar to the process used in example 5, additional microsize NaCl saltwas added in certain percentage to make the casting slurry. Aftermilling, the homogeneous ink will be used to cast the 3D structuredsubstrate. After drying and peeling off, the film is then soaked indistilled water for a period of time, followed by drying in vacuum oven.By controlling the amount of NaCl addition (as pore former), the filmporosity can be controlled to up to 95%.

Example 8. 3D Lithium Anode

3D lithium anode was made by laminating a layer of 3D structure made inExample 6 on top of lithium foil. This 3D lithium anode was punched in adisk size. Symmetric cells (cell type A) were made with two pieces of 3Dlithium anode with Celgard 2400 separator and 1M LiPF₆ in DMC/EC (1:1)electrolyte. The cells were charged and discharged at 0.5 mA/cm² for 2hours. As comparison, symmetric cells (cell type B) were made with twopieces of metallic lithium disk with Celgard 2400 separator and 1M LiPF₆in DMC/EC (1:1) electrolyte and were cycled at 0.5 mA/cm² for 2 hours.The cell type A can last over a few hundred cycles before exhibitingincreasing of cell voltage, while cell type B can only last for lessthan 15 cycles before cells develop erratic voltage profiles. Cell typeA can also be cycled stably at high current density of 10 mA/cm² whilecell type B cannot be cycled at such high current density for more than10 cycles.

3D lithium anode with 3D structure with conductivity gradient can notonly facilitate the bottom-up lithium deposition, but also block thelithium dendrite growth—extending the cycle life lithium metalbatteries.

Example 9. Rechargeable Lithium Metal Cells

3D structure with conductivity gradient was made with Zn doped copperpowder and N-doped carbon black. 3D lithium anode can then be made bylaminating this 3D film on top of lithium metal or lithium metal oncopper current collector. Disk of 3D lithium anode can be punched out.Coin cells can be made with NMC 532 cathode, Celgard 2400 separator and3D lithium anode. Conventional electrolyte or any proper non-aqueouselectrolyte can be used to make the coin cell. The cell was then becharged and discharged at C/5, C/2 and 5 C rate. Conventional coin cellswith conventional lithium foil anode were also fabricated and cycled.The cells with 3D lithium anode showed much longer cycle life and higherrate capability than the conventional cells.

What is claimed is:
 1. An anode, comprising: an electrically conductivefirst layer comprising: a porous, three-dimensional structure having afirst end and a second end; particles comprising a subset of particlesthat are distributed at a higher concentration toward the first end thanthe second end such that an electrical conductivity gradient is formedbetween the first and second ends; and a second layer comprising lithiummetal, wherein the second layer is coupled to the first end of the firstlayer.
 2. The anode of claim 1, wherein the second end is configured tocouple with a separator and an electrolyte solution.
 3. The anode ofclaim 1, wherein the first layer comprises a polymer.
 4. The anode ofclaim 1, wherein the first layer comprises: a polymer.
 5. The anode ofclaim 1, wherein the particles comprise at least one of copper andnickel and are surface coated with at least one of tin, zinc, argon,bismuth, indium, gallium, aluminum, nitrogen, phosphorous, silicon, andgermanium.
 6. The anode of claim 1, wherein the particles are doped withat least one of: tin, zinc, argon, bismuth, indium, gallium, aluminum,nitrogen, phosphorous, silicon, germanium.
 7. The anode of claim 1,further comprising a metal current collector, wherein the currentcollector comprises at least one of copper, nickel, and stainless steel.8. The anode of claim 1, further comprising a metal current collector,wherein the current collector comprises at least one of copper, nickel,and stainless steel.
 9. The anode of claim 1, wherein the second layercomprises pure metallic lithium.
 10. The anode of claim 1, wherein thefirst layer comprises a porosity between 60 percent and 90 percent. 11.The anode of claim 1, further comprising a third layer, the third layerbeing a metal current collector comprising at least one of copper,nickel and stainless steel, and wherein the third layer is coupled tothe second layer.
 12. A battery, comprising: an anode, comprising: anelectrically conductive first layer comprising: a porous,three-dimensional structure having a first end and a second end;particles comprising a subset of particles, said subset having a highelectrical conductivity that are distributed at a higher concentrationtoward the first end than the second end; and a second layer comprisinglithium metal, wherein the second layer is coupled to the first end ofthe first layer.
 13. The battery of claim 12, further comprising: acathode located in closer proximity to the second end than to the firstend; a separator; and an electrolyte; wherein the separator andelectrolyte are located in between the cathode and the anode.
 14. Thebattery of claim 12, wherein the first layer of the anode iselectrically conductive and comprises a higher electrical conductivitytoward the first end than the second end.
 15. The battery of claim 14,wherein the first layer of the anode comprises a polymer.
 16. Thebattery of claim 14, wherein the first layer of the anode comprises: apolymer; and particles comprising at least one of carbon and metal,wherein the particles comprise particles with high electronicconductivity that are distributed at a higher concentration toward thefirst end than at the second end.
 17. The battery of claim 16, whereinthe particles in the anode are carbon particles and are doped with atleast one of: tin, zinc, argon, bismuth, indium, gallium, aluminum,nitrogen, phosphorous, silicon, germanium.
 18. The battery of claim 16,wherein the particles in the anode are metal particles and are surfacecoated with at least one of: tin, zinc, argon, bismuth, indium, gallium,aluminum, nitrogen, phosphorous, silicon, germanium.
 19. The battery ofclaim 17, further comprising a metal current collector coupled to thesecond layer of the anode, wherein the current collector comprises atleast one of copper, nickel, and stainless steel.
 20. The battery ofclaim 16, further comprising a third layer of metal current collectorcoupled to the second layer of the anode, wherein the current collectorcomprises at least one of copper, nickel, and stainless steel.
 21. Thebattery of claim 16, wherein the second layer of the anode comprisespure metallic lithium.
 22. The battery of claim 16, wherein the firstlayer of the anode comprises a porosity between 60 percent and 90percent.
 23. The battery of claim 14, wherein the anode is furthercomprised of a third layer, the third layer being a metal currentcollector coupled to the second layer, and wherein the current collectorcomprises at least one of copper, nickel, and stainless steel.